Polypyrrole nanoparticles as a thermal transducer of NIR radiation in hot-melt adhesives

Polypyrrole (PPy), like other conducting polymers, has a broad absorption band in the near infrared (NIR) with no evidence of fluorescence emission. We describe the preparation of PPy-EVA blends as potential hot-melt adhesives that can be activated by irradiation with NIR light. The PPy content needed to act as a thermal transducer of NIR radiation is much lower than that needed for conductivity. Blends were prepared in two ways: by blending sterically-stabilized 50 nm diameter PPy particles in water with a dispersion of 800 nm diameter ethylene-vinyl acetate copolymer (EVA) particles, and by synthesizing PPy-coated EVA core-shell particles by precipitation polymerization in water. The PPy nanoparticles and the PPy-coated EVA core-shell particles could be purified by sedimentation followed by redispersion in water to remove Fe salts. Films prepared from these particles, containing 0.1-0.5 wt% PPy, showed a strong NIR absorbance in the range of our spectrometer (700-1100 nm) with a weaker absorbance in the visible region. Photothermal radiometry (PTR) measurements indicate that these blends show good promise as potential NIR-activated adhesives, which are essentially transparent to the eye.